1. Field of the Invention
The present invention relates generally to voltage mode class D topologies and, more particularly, to high efficiency voltage mode class D amplifiers and wireless energy transfer systems.
2. Description of the Related Art
Recently, there have been many developments in wireless power transmission systems (also referred to as an “energy transfer systems”) using highly resonant electromagnetic induction. In general, such systems include a power source and transmitting coil as well as a receiving coil connected to the device to be power (i.e., the load). The architecture for wireless power transmission systems is centered on the use of coils to generate a high frequency alternating magnetic field that is used to transfer energy from the source to the device. The power source will deliver energy in the form of voltage and current to the transmitting coil that will create a magnetic field around the coil that changes as the applied voltage and current changes. Electromagnetic waves will travel from the coil, through free space to a receiving coil coupled to the load. As the electromagnetic waves pass by and sweep the receiving coil, a current is induced in the receiving coil that is proportional to the energy that the antenna captures.
When the source and load are coupled during wireless power transmission, the resulting configuration effectively forms a transformer with a low coupling coefficient. This resulting transformer has a leakage inductance that can be significantly larger than the magnetizing inductance. An analysis of the transformer model under these conditions reveals that the primary side leakage inductance almost solely determines the efficiency of energy transfer. To overcome the leakage inductance, some systems use resonance to increase the voltage across the leakage inductance and hence the magnetizing inductance with resulting increase in power delivery.
One conventional wireless energy transfer topology uses a traditional voltage mode class D (“VMCD”) amplifier in wireless energy transfer system. FIG. 1 illustrates a circuit diagram for a VMCD amplifier. As shown, the VMCD amplifier 100 includes a power amplifier 110 and load 120. The power amplifier 110 includes two transistors 111 and 112 that are coupled in series between a voltage source VDD and ground. The two transistors 111 and 112 are driven 180° out of phase to form a half bridge topology. Conventionally, the transistors 111 and 112 can be enhancement mode, n-channel MOSFETs, for example. Furthermore, power amplifier 110 includes a first capacitor 113 and inductor 114 that are coupled in series with load 120 to form a resonant tuning circuit. In this conventional design, the power amplifier 110 tunes the load to have a resonance at the same frequency as operation of the amplifier 110. Despite zero current switching (“ZCS”), the power amplifier 110 still experiences high losses due to the output capacitance COSS of the transistors 111 and 112 each time a voltage transition occurs. As the frequency increases, the losses also increase proportionally.
To overcome these problems, existing circuits have added a matching network to the load 120 to make the load 120 appear inductive to the power amplifier 110. For example, FIG. 2 illustrates a modified circuit of VMCD amplifier 100 illustrated in FIG. 1, but includes a matching network. As shown, VMCD system 200 includes transistors 211 and 212 and further includes inductor 213 and a first capacitor 214 coupled in parallel with transistor 212. Furthermore, a second capacitor 215 is connected in series with load 220 to form a load resonant circuit 210. Due to high switching frequency and device output capacitance COSS, the load resonant circuit configuration (i.e., load 220 and second capacitor 215) must be tuned to be inductive at operating frequency, and, therefore, allow zero voltage switching (ZVS) and corresponding reduction in output capacitance COSS losses. In design, this tuning can lead to operation of the power amplifier 110 above resonance with decrease in coil transmission efficiency. Although the amplifier will operate with reduced losses (i.e., require less cooling), the improved amplifier efficiency does not offset the reduced coil transmission efficiency.
The matching circuit (inductor 213 and capacitor 214) functions to increase the voltage to the load resonant circuit (capacitor 215 and load 220), which can be advantageous when limits are placed on the input voltage magnitude, given that the average voltage at the output of the amplifier (switch-node) is half the supply voltage VDD. However, the matching inductor will carry the full current of the load and thus will have significant losses. Furthermore, the circuit is sensitive to load resistance variation as the matching network becomes an integral part of the tuned resonant circuit, which can shift the ideal operating inductance point to maintain proper ZVS.
Accordingly, a high efficiency VMCD amplifier and energy transfer system is desired that is preferably low profile for both the source and device units, easy to use, highly robust to changes in operating conditions, and does not require forced air cooling or a heat sink.